Almost all common projection devices are based on one of the basic image modulator devices. Where the old technology using Cathode Ray Tubes (CRT) is almost completely outdated, the liquid crystal light valves (LCDs) and is the digital micro-mirror devices (DMDs) currently are used in most of the projectors. During the last years, a new liquid crystal technology, the so-called Liquid Crystal on Silicon (LCOS) has emerged. An LCOS chip is a reflective chip, where the liquid crystal basically only has half the thickness of a normal LCD. After transmission through the light modulating layer, the light is reflected and returns through the liquid crystal, so that the total light path in the liquid crystal is identical to the passage through a transmissive LCD. Light with altered polarization is split from the rest and is displayed on a display screen.
Analogous to the situation with normal LCDs, motion artefacts called smearing effects are introduced in LCOS. As described by Jun Someya (Information Display 10/05, p 12-16), this motion blur can be understood by a combination of two phenomena.
The first one (see FIG. 1) is specifically related to the nature of the liquid-crystal valves. If one visualizes the light intensity i (axis 12) of the liquid crystal as a function of time t (axis 11), it is clear from the graph in FIG. 1 that the response curve 13 is slow, and takes a certain time period Δt to go from full black to full white. This time period can be split into two components, a delay time Δt1 during which the crystal starts to realize that the applied driving voltage has changed, and the rise time Δt2 which is needed for the liquid crystals to rotate. If one then considers moving image content, it is evident that a comet-like tail will be present, as the crystals always come slightly behind. During the timeframe 14 in which one image (frame) is displayed, the transition from 0 to 1 (e.g. black to white) takes almost the entire period. If the pixel should be black again in the next frame, it will again take more than half of the frame before the signal starts approaching 0. The slow response time of the liquid crystal cells is noticed when switching from black to white or vice versa, or from one grey level to another.
A second effect (which is not limited to liquid crystal valves) is the so-called sample and hold effect. If a point is moving fast across the image it is displayed at one certain position x during a complete frame. During the next frame, it is located somewhere else. The human eye follows the movement in a smooth way, but the illuminated pixel makes a jumping movement. On the is retina, the image and the eye attention point do not coincide and this causes blur, which is essentially the difference between the eye movement curve and the temporal position during one frame. One approach to reduce this problem has been described by Seiko Epson Corporation in EP-1575306 and is named scrolling illumination. Basically, what is does is blocking the light during a certain percentage of the frame (e.g. ½) so that during the delay time Δt1 and a part of the rise time Δt2, the light modulator is dark, e.g. not illuminated (no light reaches the light modulator). Only towards the end of a frame period 14, e.g. during the timeframe 15 as illustrated in FIG. 1, the light hits the light modulator.
In EP-1575306, the scrolling illumination is introduced by means of a rotating polygon prism 16, as illustrated in FIG. 2, which is in this case has a square base surface.
In FIG. 2, the time axis goes from top to bottom (time t0 time t0+P), and the bottom polygon position is identical to the top one, i.e. what is illustrated corresponds to a time period 14, i.e. 1 image frame.
The polygon prism 16 is rotated in such a way that that a parallel light bundle 17 is displaced with respect to its initial axis, by refraction in the material component, e.g. glass component, of the polygon prism 16. The displaced light spot hits a different zone 18 on a light modulator panel for each position of the prism 16. If at the position where the bundle 17 hits the light modulator panel a light valve is situated, the spot is scrolled across it (in the example illustrated e.g. from bottom to top, where it jumps towards the bottom again as the initial light ray sees the edge of the prism 16). It is evident that this polygon prism 16 can also be a hexagon or an octagon or any arbitrary symmetric polygon.
Typically, a light modulator panel, e.g. a liquid crystal light valve panel, of low or moderate resolution (whether it is reflective or transmissive) is ‘written’ row by row (or series of rows by series of rows), e.g. starting from the bottom and going to the top. This means that when image Io is written at time t0, this actually starts happening e.g. at the bottom of the liquid crystal light valve panel, but only happens at time t=t0+P−Δt the top of the liquid crystal light valve panel, where P corresponds to the time period between two subsequent images (refresh rate), and where Δt is a small time period. Analogously this writing process can be performed from top to bottom. Optimal performance can be seen when the scrolling light band is synchronized with the signal processing in such a way that the light band 18 is displayed where the image 19 has been written on the liquid crystal about half a period ago, as illustrated in FIG. 3. Then, only during timeframe 15 (occurring on these pixels), i.e. the end of the transition period, as illustrated in FIG. 1, the light is displayed, and the transition is no longer visible to the same amount as before. Scrolling illumination thus reduces blur.
The above solution for providing scrolling illumination, as illustrated in FIG. 2, comprises a rotating prism. Alternative solutions may comprise a rotatable drum comprising holographic elements disposed around the circumference of the drum or a rotatable disk having holographic stripes in a radial pattern on a surface of the disk, as described in WO 2003/055231, or a dynamic filter having a first region for transmitting a first portion of a light beam and a second region for rejecting a second portion of the light beam, as described in EP-1274256.
The LCOS technology enables light valve manufacturers to reduce the pixel size dramatically with respect to other light valves such as liquid crystals, as the electronic components can be positioned behind the silicon mirror which forms part of the LCOS light valve. As the pixel size can be dramatically reduced (values of 3.5 and 5 micron have been mentioned in future development plans by several companies such as e.g. Sony), the resolution of light modulator panels is inversely proportionally increased, leading to 1″ diagonal light modulator panels with a resolution of e.g. 4000×2000 pixels. These pixels are no longer addressed the same way as the pixels of prior art moderate or low resolution light modulator panels, but other principles are used in high resolution panels 20. One such addressing scheme is demonstrated in FIG. 4, the addressing scheme involving simultaneous addressing of four quadrants 21. In this case, the light valves at the centre of the light modulator panel 20 are first altered (row by row in directions 22), after which the zones of alteration spread towards the top and bottom, in directions 23, of the respective quadrants 21 of the light modulator panel 20. As a consequence the known illumination methods with a scrolling light band as described above are no longer applicable.
There is a need for other illumination solutions, adapted to the new driving schemes where data is synchronously written to different zones of light modulator panels.